How can you by law be forced to pay money for a kid that's not yours?

And if that really is the case, what does it say, whether American or otherwise, about the "fairness" or more precisely lackthereof, of such law in America and otherwise, in general?

And that man couldn't in any way try to appeal it and prove that it isn't his child for which he is paying?

Or maybe worse - there is some kind of powerful evil conspiracy behind it, and the system overall is unfortunately organized in such a way that it isn't fair and that you don't really have a choice behind it, right?